In recent years, a silicon photonics technique has been developed. The silicon photonics technique is a technique achieving a semiconductor device serving as an optical communication module by forming a transmission line for an optical signal using silicon as a material on a semiconductor substrate and integrating various optical devices and electronic devices formed from the transmission line for an optical signal. Such semiconductor devices includes the one having an optical waveguide made of a semiconductor layer formed on a base body via an insulation layer as the transmission line for an optical signal, and an insulation film formed on the insulation layer so as to cover the optical waveguide. At this time, the optical waveguide functions as a core layer, and the insulation layer and the insulation film function as cladding layers.
Further, such semiconductor devices include the one having a photoelectric conversion portion for converting an optical signal to an electric signal. In addition, such semiconductor devices include the one provided with a light receiver including a semiconductor layer made of germanium as the photoelectric conversion portion in order to detect near-infrared light having a wavelength up to about 1.6 μm that is a communication wavelength band.
Japanese Patent Application Laid-Open Publication No. 2011-181874 (Patent Document 1) discloses a technique for a germanium optical receiver provided with a first germanium layer formed on a silicon layer, a second germanium layer formed on the first germanium layer, and a silicon cap layer formed on the second germanium layer.